1. Field of the Invention
The present invention relates generally to marine seismic prospecting and, more particularly, to a method for combining dual sensor seismic data to attenuate water column reverberations in the case in which one of the signals contains significantly more random noise than the other.
2. Description of the Related Art
Marine seismic prospecting is a method for taking seismic profiles of underwater subterranean land formations. In marine seismic prospecting, a seismic survey ship is equipped with at least one energy source and at least one receiver. An energy source is designed to produce compressional waves that propagate through the water and into the subterranean formations. As the compressional waves propagate through the subterranean formations, they strike interfaces between formations, commonly referred to as strata, and reflect back through the earth and water to a receiver. The receiver is designed to detect the reflected compressional waves. The receiver typically converts the detected waves into electrical signals which are later processed into an image that provides information about the structure of the subterranean formations.
Different energy sources may be used to generate acoustic waves in marine applications. Presently, one of the most common marine energy sources is an air gun that discharges air under very high pressure into the water. The discharged air forms an energy pulse which propagates through the water and the underlying subterranean formations. Another marine energy source frequently used is a marine vibrator. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies.
Similarly, different receivers may be used to detect reflected acoustic waves. The receivers most commonly used in marine seismic prospecting are hydrophones. Hydrophones detect and convert pressure waves into electrical signals that are used for analog or digital processing. The most common type of hydrophone includes a piezoelectric element which converts pressure into electrical signals. Another receiver sometimes used in a marine setting is a geophone. Geophones detect particle velocity. Geophones are typically used in land operations where the geophones are anchored to the ground to maintain correspondence of geophone motion to ground motion. In marine applications, however, anchoring the geophones is difficult. Typically, therefore, a gimbal mechanism is used to orient the geophone vertically for proper operation. Alternatively, accelerometers or displacement detectors may be used.
Hydrophones are typically mounted on a long streamer which is towed behind the survey ship at a depth of tens of feet. Alternatively, marine seismic prospecting may combine different types of receivers. For instance, in dual sensor ocean bottom cable seismic recording, a combination of hydrophones and geophones are deployed on the marine bottom. Typically, miles of bottom cable are deployed in a planned pattern such as a single line or a plurality of substantially parallel lines.
The use of ocean bottom cables is particularly effective in obtaining full three dimensional coverage in areas too shallow or too congested with obstacles for gathering seismic data with a towed streamer. While the ocean bottom cable technique allows access to areas denied to the towed streamer method, an additional, unwanted "ghost" reflection from the air water interface, along with subsequent reverberations, occurs for each primary reflection wave. The time delay between the primary reflection signal and the ghost reflection signal is greater with the ocean bottom cable method than with the towed streamer method because the detectors are farther removed from the air-water interface in the ocean bottom cable method, except in shallow water.
Two basic approaches have been proposed for eliminating the ghost reflection. The first approach involves recording signals from detectors at different depths and then performing a wavefield separation. The second approach utilizes co-located pairs of pressure and velocity detectors, as in, for example, U.S. Pat. No. 2,757,356, "Method and Apparatus for Canceling Reverberations in Water Layers", issued to Haggerty. This second approach capitalizes upon the fact that pressure and velocity detectors generate signals which are the same polarity for upward travelling waves but are of opposite polarity for downward travelling waves, that is, the ghost reflections. This indicates that the two signals can be properly summed to eliminate the unwanted reverberations associated with each reflection. In the frequency domain, this relationship expresses itself in the complimentary amplitude spectra of the two sensors. When the signals are properly summed, a smooth amplitude spectrum results.
U.S. Pat. No. 4,979,150, issued to present inventor Barr, assigned to the assignee of the present invention, and entitled "Method for Attenuation of Water-Column Reverberations" describes a dual sensor ocean bottom cable method for attenuating the unwanted water column reverberations associated with each reflection signal in the seismic data by combining the pressure and velocity signals recorded at each receiver station. The pressure and velocity signals are scaled and summed, with a scale factor dependent upon the acoustic impedance of the water and the earth material of the ocean bottom. Proper combination of the pressure and velocity signals, in order to remove the component of the signal representing energy which is trapped in the water layer, can only be performed after scaling the velocity signal by a scale factor S given by ##EQU1## where R is the ocean bottom reflectivity. Thus the scale factor requires determining the ocean bottom reflectivity, which depends upon the acoustic impedance of the bottom material.
In the prior art, a calibration survey has been used to estimate the ocean bottom reflectivity R. In the weight-and-sum method of U.S. Pat. No. 4,979,150 described above, an estimate of the ocean bottom reflectivity is made by collecting separate reference information, generated by shooting a small seismic source directly over the receivers. The collection of this survey data requires additional time and cost beyond the data acquisition phase of the survey.
U.S. Pat. No. 5,365,492, issued to Dragoset, Jr., assigned to the assignee of the present invention, and entitled "Method for Reverberation Suppression", describes a dual sensor ocean bottom cable method for suppressing water column reverberations directly from the production dual sensor seismic data rather than from additional calibration data. Additionally, this weight-and-sum method works in the presence of random noise in the velocity signal. The noise is adaptively estimated and subtracted from the velocity signal. The velocity and pressure signals are summed with trial scale factors and the sums are autocorrelated. A varimax function is computed for each autocorrelation and used to select the best scale factor.
U.S. Pat. Nos. 5,396,472 and 5,524,100, both issued to Paffenholz, assigned to the assignee of the present invention, and both entitled "Method for Deriving Water Bottom Reflectivity in Dual Sensor Seismic Surveys", describe a dual sensor ocean bottom cable method for estimating the ocean bottom reflectivity directly from the production data rather than from calibration data, using inverse Backus filters. In this method, time windowed common receiver station hydrophone and geophone traces are added together after correcting for any differences in transduction constants. Inverse Backus filters are then applied to these summed traces with a range of ocean bottom reflection values. The estimated reflectivity value is that which minimizes the energy in the resulting filtered traces, because this value has done the best job of eliminating the undesired water column reverberations. An optimization procedure determines which trial value for ocean bottom reflectivity minimizes the spectral power. Improved attenuation of water-column reverberations over previous methods results.
U.S. patent application No. 08/871,505, filed Jun. 9, 1997 by present inventor Barr along with Paffenholz and Chambers, assigned to the assignee of the present invention, and entitled "Method for Deriving Surface Consistent Reflectivity Map from Dual Sensor Seismic Data", describes a method which allows the determination of ocean bottom reflectivities at all the source and receiver locations. Since the acoustic impedance of the bottom material, and hence the ocean bottom reflectivity, can vary among different source and receiver locations, the scale factor can vary at different locations too. A "surface consistent" map of ocean bottom reflectivities gives the different values at different locations.
Since the introduction of the dual sensor method in 1989, data has been recorded in many areas in which the vertical geophone traces contain strong random noise. Such noise compromises the quality of the prestack combined data. In some cases, the noise has been so severe that it has compromised the quality of the final imaged seismic data. In at least one seismic survey area of the world, the hydrophone data were noisier than the geophone data, but this has occurred only rarely.
When the hydrophone and vertical geophone data are combined using the weight-and-sum method, using either calibration shooting or Varimax derived scalars, the noise problem has typically been aggravated. In areas where the ocean bottom reflection coefficient is positive, which occurs in the vast majority of survey areas, the scalar that is multiplied times the geophone trace prior to summing with the hydrophone trace, is significantly larger than one. Thus a greater portion of the noisy geophone trace is used in the combined trace.
With the Backus filter method of combining dual sensor traces, the signal-to-noise ratio of the combined trace is improved relative to the weight-and-sum method because, regardless of ocean bottom reflectivity, equal portions of the geophone and hydrophone traces are used. However, in some cases, the signal-to-noise ratio of the geophone trace is so poor that the quality of the combined trace is still adversely affected.